Microelectronic devices, micromechanical devices, and other devices with microfeatures are typically formed by constructing several layers of components on a workpiece. In the case of microelectronic devices, a plurality of dies are fabricated on a single workpiece, and each die generally includes an integrated circuit and a plurality of bond-pads coupled to the integrated circuit. The dies are separated from each other and packaged to form individual microelectronic devices that can be attached to modules or installed in other products.
One aspect of fabricating and packaging such dies is forming interconnects that electrically couple conductive components located in different layers. In some applications, it may be desirable to form interconnects that extend completely through the dies or through a significant portion of the dies. Such interconnects electrically couple bond-pads or other conductive elements proximate to one side of the dies to conductive elements proximate to the other side of the dies. Through-wafer interconnects, for example, are constructed by forming deep vias from the backside of the wafer to bond-pads on the front side of the wafer. The vias are often blind vias in that they are closed at one end. The blind vias are then filled with a conductive material. After further processing the wafer, it is eventually thinned to reduce the thickness of the final dies. Solder balls or other external electrical contacts are subsequently attached to the through-wafer interconnects at the backside of the wafer.
One concern of forming through-wafer interconnects in blind vias is that it is difficult to form such deep, narrow holes in the wafer. The blind vias are often formed at a stage when the workpieces are approximately 750-1,500 μm thick. The blind vias can be formed by etching the holes through a significant portion of the workpiece, but etching deep, narrow holes requires a significant amount of time. Moreover, the depth of the holes is difficult to control and the etchant may damage features on the backside of the workpiece.
The blind vias are also formed by laser drilling holes into the workpiece. Laser drilling deep, narrow holes through the workpiece is not practical in several applications. First, it is difficult to control the depth of the holes. More specifically, the bond-pad may not be exposed if the hole is not deep enough, or the bond-pad may be ablated if the hole is drilled too deep. Second, laser drilling deep into the workpiece produces large heat-affected zones that may affect neighboring structures within the wafer and slag that is difficult to clean. Therefore, etching or laser drilling such deep, high aspect ratio holes in a workpiece at this stage of the fabrication process may not be practical in many applications.
Another concern of forming through-wafer interconnects is that it is difficult to fill deep, narrow blind vias. Vapor deposition processes, for example, may not uniformly cover the sidewalls in such holes, and this may cause the openings of the holes to be “pinched-off” before the holes are filled with the conductive material. The resulting interconnects may have voids. Plating processes may also produce voids because the seed layers have similar nonuniformities that cause the plating rate to be higher at the openings than deep within the vias.
Furthermore, it is not feasible to further thin the workpieces before forming the interconnects because the workpiece may not have sufficient structural integrity to withstand the automatic handling equipment used in subsequent processes. For example, very thin wafers easily flex and will partially conform to vacuum chunks used in handling the wafer. Such thin wafers are also very delicate and are easily damaged or broken. Therefore, there is a need to more effectively form blind vias and other deep holes in microfeature workpieces.